The expression "visual display of moving targets" is understood to mean visualizing without major gaps in the useful radial-velocity range of targets, the moving echoes belonging to a particular Doppler frequency band while eliminating as far as possible the fluctuating fixed echoes and their fluctuations caused by the possible rotation of the antenna as well as by low-velocity clutter.
In order to obtain this better visual display the repetition frequency and thus the duration of the pulse-repetition period of the Doppler radar in question is varied.
It is known that in a Doppler radar certain velocities are said to be blind for certain values of the pulse-repetition period. By varying the value of this repetition period from one occurrence to the next, certain targets which were invisible reappear while the fixed echoes continue to be eliminated no matter what the value of the repetition period. However, this variation of the repetition period also has the effect of significantly reducing the degree of attenuation of the low Doppler frequencies, in particular those close to zero.
According to a known system, the spread of the spectrum of the fixed echoes caused by the variation of the repetition frequency in electromagnetic detection equipment is compensated by forming the useful signal from a target on the basis of the difference between the signals successively received from that target and by forming a weighted sum of these signals with weighting factors selected in such a way as to return the signals to the values which they would have had in the absence of any variation in the repetition frequency.
In such a system, signals in relative quadrature from two lock-in detectors are preferably digitally sampled. In each of the channels in question, the so-called sine channel and the so-called cosine channel, two samples are taken per echo and are stored in such a way that samples corresponding to three successive recurrences are simultaneously available.
The differences between the three thus defined signals are applied to a weighting device which comprises two multipliers with variable coefficients which are a function of the recurrence in question. The signals from these weighting devices are summed and then processed in known manner in a filtering and detection device.
Thus, in that prior-art process and system, weighting is performed on two different signals which must be delayed one relative to the other so that the weighted signals corresponding to the same distance can be summed. This makes it necessary to have a supplementary memory in each channel, which increases equipment costs. Moreover, it has been found that the desired rejection of low-frequency echoes was not very satisfactory.